This invention relates generally to controls for gas appliances, and more particularly to a controller for controlling power to an igniter for a gas furnace.
Most gas furnaces manufactured today include some type of electronic ignition system and, in particular, hot surface igniters that produce high temperatures (e.g., 2000+ degrees Fahrenheit) for burner ignition within the furnace. The hot surface igniter, when activated, ignites gas flow of a main burner of the furnace without the need for a pilot light. These electronic ignition systems reduce gas consumption and increase the efficiency of the furnace, thereby increasing the efficiency of the HVAC system to which they are connected. Further, gas ignition is provided without the need for a continuously burning pilot light or flame, thereby reducing the possibility of a dangerous condition occurring (e.g., ignition of lint or other combustibles near the pilot flame) and providing an easier means for igniting the furnace (e.g., no need to use a match to reignite the pilot flame).
Several different types of hot surface igniters exist for use with gas furnaces. The most common types include silicon nitride igniters, silicon carbide igniters, and mini silicon carbide igniters. Further, hot surface igniters may be constructed of different materials including aluminum nitride, silicon nitride, silicon carbide, boron carbide, tungsten disilicide, tungsten carbide, and mixtures thereof. More and more, silicon nitride igniters are used and are replacing other types of hot surfaces igniters because of their durability and consistent performance, thereby improving efficiency and providing longer useful life. Further, these silicon nitride igniters are capable of operation under many different and varied conditions, including, for example, when exposed to water, condensation, bleach and other contaminants.
However, due to the operating characteristics of the materials used in these igniters (and in particular silicon nitride), as well as the extreme variations in temperature experienced by these igniters, proper control of the power supply to the silicon nitride igniter is required in order to prevent a direct current component across the igniter. This is needed in order to minimize material migration of igniter elements, which migration may result from the igniter brazing material (e.g., silver) migrating between the positive igniter electrode and negative igniter electrode, thereby causing a short across the electrodes and failure of the igniter. Minimizing material migration by controlling the power supply (e.g., controlling power level and switching of the power supply) to the igniter extends the igniter life.
Thus, it is desirable to regulate power to silicon nitride hot surface igniters to maintain proper operating conditions, such as a proper power level to maintain a proper temperature to provide gas ignition. As recognized by the inventor hereof, a control for controlling power to a silicon nitride hot surface igniter should respond to changes in line voltage to provide a constant effective voltage across the igniter. Known control devices and methods for use in connection with silicon nitride igniters, and in general resistive heating elements, are typically integral half-cycle controls (IHC), proportional controls or phase controls. These devices and methods usually switch at or below the main power frequency and typically use a triac as a switching element. These devices and methods usually provide poor power factor, flicker and high harmonic currents, which may result in interference (e.g., EMI). Further, IHCs produce sub-harmonics. Therefore, these devices and methods must be filtered to reduce harmonics on the power line. New regulations for flicker and current harmonics will likely soon require improvement of the power quality of control devices for controlling power to these types of igniters.
The present invention provides an igniter controller for controlling power to a hot surface igniter that responds to changes in line voltage to the igniter to provide a constant and effective voltage across the igniter. The igniter controller uses asynchronous pulse width modulation that switches at a frequency above the audible range, thereby reducing potential interference (e.g., EMI). More particularly, the igniter controller uses a single switching transistor in series with the igniter that is pulse width modulated and that provides current flow in both directions to the igniter using a full wave rectifier bridge (i.e., switching of alternating current (AC)).
Specifically, in one embodiment of the present invention an igniter controller includes means for determining the voltage across an igniter, means for comparing the voltage across the igniter to a reference voltage, and means for selectively changing an alternating current (AC) for powering the igniter and based upon the comparison. The means for selectively changing may include means for switching the AC across the igniter. The igniter controller may further include means for setting or varying the reference voltage.
In another embodiment of the present invention an igniter controller for use with a hot surface igniter includes a filter circuit for filtering an alternating current (AC) power source, a switching transistor for switching the filtered AC power across the hot surface igniter, a full wave rectifier bridge for rectifying the AC power source to provide rectified AC power for switching the switching transistor, and a pulse width modulation and error control circuit for comparing a reference voltage to the voltage across the hot surface igniter to control the switching transistor for switching the filtered AC power across the hot surface igniter. The igniter controller further may include a variable resistor for setting the reference voltage.
Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiments of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention.